Discussion topic:
You have read the about homogeneity and heterogeneity corrections. Talk to your preceptors and/or clinical instructors about the effect each can have on a treatment plan. Does your site use heterogeneity or homogeneity? Why or why not?
Explain what a CT to density table is, how the planning system uses it, and why it is so important.
Discussion post:
Tissue hetero and homogeneity are two aspects of treatment planning that are of particular importance when developing a treatment plan. Differences in tissues can have major effects of a treatment plan. Heterogeneities can affect radiation beam absorption or can cause increased scatter among the photons passing through the patient resulting is altered dose distribution.1 At my clinic we use homogeneity corrections in the form of density overrides to assign areas of concern a homogeneous density. The main use for density overrides is around artificial implants such as hip replacements in heterotopic hip treatments, tissue expanders in chest wall treatments, around fillings and dental implants in head and neck patients.1
The main form of correction used at my clinic is heterogeneity correction by the treatment planning system. Heterogeneity correction is used because it gives a more accurate representation of the treatment dose distribution. To show the difference in correction methods my clinical instructor assigned an AP and PA beam treating a laterally located middle lobe right lung tumor. The isodose distribution on a homogeneity corrected plan was hot on the lateral side near the rib margin and underdosed medially where the ant and post separation was thicker. This was due to the homogeneous calculation method that recognized the lung tissue as the same density and thus receiving less dose in the thickest part of the patient. Insertion of a 30 degree wedge evened out the dose distribution and the plan looked good but we knew this was not accurate because the planning system didn’t account for the low density of the lung tissue. Next, we changed to a heterogeneous dose calculation method that more accurately represented the patient’s anatomy. Dose distribution was cool through the rib margin due to the dense bone and very hot medially anterior and posterior due to the low density if the lungs. The plan looked terrible but we knew this was an accurate representation, the plan will require IMRT to provide optimal dose distribution.
A CT density table is a method of converting Hounsfield units (HU) to an electron density (ED).1 To accomplish this a phantom with known densities is scanned and a unique CT to ED table is created for a CT simulation scanner. The data from the CT to ED table is loaded into the treatment planning system which assigns an attention value to each voxel based on its HU density. If a patient is scanned on a CT simulator that does not have a CT to ED table than the data from that scan will not be accurate and dose calculation errors can result. It was explained to me by one of our physicists that the difference between simulators is typically minimal but nonetheless we strive to accuracy and perfection.1
References
You have read the about homogeneity and heterogeneity corrections. Talk to your preceptors and/or clinical instructors about the effect each can have on a treatment plan. Does your site use heterogeneity or homogeneity? Why or why not?
Explain what a CT to density table is, how the planning system uses it, and why it is so important.
Discussion post:
Tissue hetero and homogeneity are two aspects of treatment planning that are of particular importance when developing a treatment plan. Differences in tissues can have major effects of a treatment plan. Heterogeneities can affect radiation beam absorption or can cause increased scatter among the photons passing through the patient resulting is altered dose distribution.1 At my clinic we use homogeneity corrections in the form of density overrides to assign areas of concern a homogeneous density. The main use for density overrides is around artificial implants such as hip replacements in heterotopic hip treatments, tissue expanders in chest wall treatments, around fillings and dental implants in head and neck patients.1
The main form of correction used at my clinic is heterogeneity correction by the treatment planning system. Heterogeneity correction is used because it gives a more accurate representation of the treatment dose distribution. To show the difference in correction methods my clinical instructor assigned an AP and PA beam treating a laterally located middle lobe right lung tumor. The isodose distribution on a homogeneity corrected plan was hot on the lateral side near the rib margin and underdosed medially where the ant and post separation was thicker. This was due to the homogeneous calculation method that recognized the lung tissue as the same density and thus receiving less dose in the thickest part of the patient. Insertion of a 30 degree wedge evened out the dose distribution and the plan looked good but we knew this was not accurate because the planning system didn’t account for the low density of the lung tissue. Next, we changed to a heterogeneous dose calculation method that more accurately represented the patient’s anatomy. Dose distribution was cool through the rib margin due to the dense bone and very hot medially anterior and posterior due to the low density if the lungs. The plan looked terrible but we knew this was an accurate representation, the plan will require IMRT to provide optimal dose distribution.
A CT density table is a method of converting Hounsfield units (HU) to an electron density (ED).1 To accomplish this a phantom with known densities is scanned and a unique CT to ED table is created for a CT simulation scanner. The data from the CT to ED table is loaded into the treatment planning system which assigns an attention value to each voxel based on its HU density. If a patient is scanned on a CT simulator that does not have a CT to ED table than the data from that scan will not be accurate and dose calculation errors can result. It was explained to me by one of our physicists that the difference between simulators is typically minimal but nonetheless we strive to accuracy and perfection.1
References
- Khan FM, Gibbons JP. Treatment planning I: isodose distributions. In: Khan FM, Gibbons JP, ed. The Physics of Radiation Oncology.5th Philadelphia, PA: Lippincott Williams & Wilkins; 2014:195-232.